Hydraulic cement compositions, and particularly those compositions used as dry-set mortars, are well established in the ceramic tile industry and are now used to install more than one-third of all the tile in the United States.
Conventional mortars generally contain Portland cement, sand, and water. The Portland cement component or an equivalent is a hydraulic cement which requires water in order to harden or cure. As a result, these mortars are not entirely self-curing because they tend to lose considerable amounts of water by evaporation into the surrounding air and also by absorption into the tile or masonry being set. Where the water loss is too great, the curing action is incomplete and the mortar becomes soft and chalky.
By increasing the proportion of water in the mortar to counteract the water loss, the mortar becomes too thin to work properly. A high water-mortar ratio also leads to cracking due to excessive shrinkage of the mortar during curing.
To insure sufficient water for hardening of the mortar, very wet conditions must be maintained on the substrate over which the mortar is placed and in the atmosphere surrounding the tiled area during the period of curing. Where non-vitreous, absorbent tiles are set, the loss of water is even greater to the porous backs of the tile. Soaking of all of the non-vitreous tile must be done before setting.
These soaking and humidity maintenance steps require special attention, add to the labor and equipment costs, and where neglected, lead to bond failure and unsatisfactory tile installations.
These conventional mortars do have many advantages including high bond strength to masonry, tile and other surfaces when cured under optimum conditions. However, in addition to aqueous maintenance, the mortars require heavy, thick and often multiple layers for setting tile which again add to the costs of tile installation.
Like the conventional mortars, the dry-set mortars contain a hydraulic cement which requires water in order to cure. The dry-set mortars, however, overcome the general troublesome problem of insuring that wet conditions be maintained during setting and curing. This is done by including certain additional components in the mortar composition which it is believed increase the viscosity of the liquid phase of the mortar mixture and improve its water-retentive properties. This prevents the dry-set mortar from losing substantial amounts of water to its surroundings during its curing stages. This property, among others, has considerably reduced the efforts and costs incurred in the installation of tile, particularly non-vitreous tiling, and has rendered dry-set mortars a great breakthrough in the adhesive industry. Dry-set type mortars have been described, for example, in U.S. Pat. Nos. 2,820,713; 2,934,932; 2,990,382; 3,030,258 and 3,243,307.
Since the original introduction of dry-set mortar compositions on the market, there has been a continuing desire to increase the standards of performance of such compositions. High standards of performance have become particularly critical due to the varied uses of these mortars.
One major problem that has occurred is that because of such high standards, one composition has not been able to be used for all purposes. It should be noted that the industry's standards referred to herein relate to the properties of the final hardened product, including its compressive strength and bond strength to tile, as well as the properties of the mortar composition during its application or installation. The properties that relate to the application of the mortar composition are referred to as the workability of the composition.
Rigorous standards of quality of performance of mortar compositions have been set by association like the Tile Council of America, Inc., under its Triangular Seal of Quality. Among the properties considered most important are (a) initial set time (pot life), (b) open time, (c) skinning, (d) sag on a vertical surface, (e) wet bond strength to tile, (f) dry bond strength to tile, and (g) compressive strength. Such standards often exceed even the performance requirements set by the American National Standards Institute (ANSI).
Initial set time, lack of open time, fast skinning over, and too much sag are shortcomings incident to the use of the mortar on the job by the tile mechanic, and are collectively referred to hereinafter as the workability of the mortar. These properties most significantly affect the efficiency and speed in which the tiles are installed. However, as it affects the efficiency of the installation, it also will affect the final properties of the installed tile.
Dry-set mortars containing varying amounts of sand were used in bonding all types of tile when they first entered the market. Then, as the standards and the requirements in the industry increased, specialized formulations were developed to obtain the high standard of properties required in each particular use. Although such literature as the Wagner patent, U.S. Pat. No. 3,030,258, describes compositions for grouting and setting all types of tile, it has been found that all dry-set compositions are not ideally suited for bonding vitreous, non-porous tile which is most often used in floor tiling. For example, all of these compositions do not consistently obtain the necessary compressive strength and shear bond strength to vitreous tile that is required in floors.
As a result, at least two distinct formulations for dry-set mortars have been developed in the industry and are now sold on the market. One type of mortar is used for vitreous and impervious tile in floors and floor coverings and is characterized as containing a reasonably coarse sand or other inorganic material as its filler component. This formulation offers satisfactory bond strength to vitreous tile and satisfactory compressive strength which is essential to support the tile which is under heavy use in flooring.
A second formulation is sold for non-vitreous or absorbent tile used for wall tiling. This formulation contains a fine sand as its filler component, usually in quantities less than that used in mortars for vitreous tile flooring. This second mortar has superior workability properties, including trowelability. This formulation has become so well accepted in the industry because of its improved workability and the fact that adequate strength can be obtained to non-vitreous tile by using no sand or with comparatively low quantities and finer grain size of sand in the mortar. Also, the need for compressive strength is considerably less for wall tiling.
Generally, with comparatively low quantities of filler, mortars achieve sufficient bond strength to non-vitreous tile, while to obtain adequate bond strength to vitreous tile, considerably more sand must be used. This additional sand, and particularly in coarser grain size, requires a sacrifice in workability of the mortar.
Sand or other fillers are inexpensive and are used also to economize the mortar as well as reduce shrinkage. The inert filler aggregate may be silica sand, crushed limestone, or other clean inert material. The inert filler aggregate component of the composition may comprise one or a combination of clean inert materials. The ultimate criteria of the component is that such component be graded to the desired grain size. Sand shall be referred to hereinafter as illustrative of the inert filler.
The terms coarse sand and fine sand as used herein are relative terms which do not carry any connotation of a sharp distinction in grain size. Generally, the grain size of the filler is described by its AFS grain fineness number, which is approximately the number of meshes per inch of that sieve which would just pass the sample if its grains were of uniform size, that is, the average of the sizes of grains in the sample. Calculation of it is according to a standard American Foundry Society method. The larger the AFS number, the finer the grain.
The most successful commercial formulations for use with vitreous floor tile have contained about 60% or more sand by weight wherein the sand has a grain fineness number of about 50. Mortar compositions for use with non-vitreous tile have generally contained about one part fine sand to one part mortar pre-mix where the sand has a grain fineness number of greater than about 50 and preferably greater than 70.
Although the industry has indicated that the vitreous tile mortars can be used for wall tile, those setting wall tile still prefer the wall type mortar as it is much more workable.